A gas turbine engine such as that used for powering an aircraft in flight includes, for example, numerous tubes for channeling various fluids through the engine during operation. Clamps are used for mounting the tubes to the engine casing, for example, at standoff brackets to accurately position the tubes and prevent their movement during operation of the engine.
Since the gas turbine engine includes various rotating components, the tubes are subject to vibratory excitation which must be controlled for preventing vibratory fatigue damage to the tubes. Conventional tube clamps used in an aircraft gas turbine engine typically have two clamp halves pivoted together at respective first ends so that the clamp halves may be opened for inserting one or more tubes between the halves, the halves then pivoted together to capture the tubes. Each clamp half includes a generally semi-circular recess which collectively surround a respective tube, and a fastener hole that extends through the tube clamp halves so that a suitable fastener, such as for example, a bolt, may be inserted through the holes, with a complimentary nut joined to the bolt for clamping together the tube clamp halves around one or more tubes contained therein. The fastener typically also extends through an engine-mounted bracket for joining the tube clamp and the tubes to the engine casing.
An example of a second prior art tube clamp design includes a base plate or lower clamp half, having a flat lower surface disposed on a support plate, a flat upper surface having an arcuate, semi-circular, first recess for receiving the tube, and a first hole spaced laterally from the first recess and extending through the base plate from the lower to upper surfaces. A capture plate or upper clamp half is positioned above the base plate and includes a lower surface facing the base plate upper surface, an upper surface, and a second hole extending through the capture plate from the lower to upper surfaces.
Clamps of the character indicated are used in aircraft construction for support of tubing in various environments, involving, for example, relatively great longitudinal displacement as in the course of wing flexure or lesser longitudinal displacements as in the case of vibratory oscillation, and elevated temperature as in the vicinity of an engine. The aircraft structure experiences high vibration levels, temperature variations, aerodynamic buffeting, and structural flexure. Often, a wear sleeve extends around the tubing, a clamp extends around the wear sleeve, and the clamp is coupled to the engine wall. The clamp is loosely coupled around the wear sleeve so that when the tubing moves relative to a wall, the wear sleeve moves axially within the clamp.
Tube clamps used within gas turbine engines are typically made of suitable metals such as for example, aluminum, stainless steel, or Inconel which are selected for use in the engine depending upon the temperature of the individual location, from relatively cool near the fan of the engine to relatively hot near the combustor and turbines. Metal tube clamps are known to abrade or chafe the tubes contained within them due to vibratory excitation of the tubes during engine operation, so that a conventional wear sleeve made of, for example, epoxy is positioned between the tube and the tube clamp to prevent undesirable wear of the tube during operation.
Because metal is known to be a poor vibration damper, metal tube clamps provide little vibratory damping of the tubes contained within them, and these tubes are subject to vibratory excitation during operation of the engine. Wear sleeves associated with these tube clamps are an additional part that must be suitably secured to the tube to prevent their liberation during operation of the engine, which is undesirable.
To combat these problems, polymer resin and fiber composite tube clamps, for example, polyimide resin and carbon fiber composite are currently being utilized in tube clamp design. Sheets or plies of composite prepreg, referred to as prepreg, are laid up into large flat plates of different thickness and cured. Clamps are then subsequently machined from these plates to meet various required configurations.
Composite tube clamps are characterized by the absence of discreet wear sleeves surrounding the tubes as would be required in metal tube clamps. A composite tube clamp will, therefore, have significant weight savings over a metal design and will also have inherent vibratory damping capability significantly greater than that which is obtainable from metal tube clamps. This damping reduces the vibratory energy in the tubes and increases the useful life of these tubes.
Composite tube clamps may be formed of conventional polymeric resins that are commercially available. For example, clamps used in the cooler regions of the engine near, for example, the fan may be made from low temperature resins like epoxy or bismaleimide (BMI). In hotter regions of the engine, polyimide resin composites such as PMR-15 polyimide matrix resin developed by the NASA Lewis Research Center and AMB-21, may be used due to their higher temperature capability. Furthermore, structural fibers may be used in a matrix for providing selective strength of the tube clamp. Conventional fibers, such as, for example, fiberglass or carbon fibers or polymeric fibers have been used in a suitable resin matrix such as those disclosed above. The fibers have been oriented at random or they have been aligned for obtaining additional strength.
These composite clamps are manufactured by laminating a plurality of prepreg sheets or plies into large flat plates of different thickness, then machining the plate into strips, and subsequently machining from these strips clamps to meet various predetermined configurations. The machining process exposes fiber ends; these machined ends of the fiber which contact the tube abrade the tube causing it to wear locally under the clamp.
Another inherent problem with a machined composite clamp is that due to its laminar design, when the clamp is installed, it frequently delaminates or cracks as a result of the load necessary to assemble it. This cracking and delamination occurs, for example, between plies in a location of maximum flatwise tension due to bending, for example, as would be caused by tightening of the fastening mechanism holding the upper and lower clamp halves against the tube. In actual use, installation failure rates of up to thirty three percent have been reported.
What is needed is a composite tube clamp that can take advantage of the weight saving properties of composite, utilizing the strength of a fiber reinforcement, thus significantly increasing the delamination/cracking resistance, while at the same time, not exposing abrasive fiber ends to create wear on a tube contained within the clamp.